This invention concerns alloy of which constituents include primarily magnesium and manganese, which alloy are particularly suitable for production of or incorporation into products in the form of pellets for administering to ruminants by deposition into the rumeno-reticular sacs.
Pellets containing magnesium and other metallic animal dietary supplements or active agents can be administered, for example by means of an oesophageal balling gun, to ruminants such that they are dissolved in the rumen juices. Such dissolution or corrosive degradation takes place over a period of several weeks so as to provide a continuous or alternatively a dosed supply of magnesium with or without other valuable metallic elements and/or active agents for assimilation by a ruminant.
In order that the amount of magnesium or active agent assimilated by the ruminant can be accurately determined and controlled it is essential that the corrosion or dissolution of the pellet can be accurately determined. It is possible to control corrosion of magnesium alloy in rumen juices by adding aluminium, copper or zinc in predetermined quantities. Such additional elements may also assist or control castability and the mechanical properties of the alloy and of a pellet cast therefrom.
A common form of pellet for administration to a ruminant comprises a hollow tubular body of a magnesium based alloy, its internal cavity containing, usuaully a plurality of tablets or a preformed body containing a biologically active agent such as an anthelmintic or a similar active drug.
In the rumen juices of a ruminant corrosion or dissolution of such an exterior magnesium alloy tube causes release of the active agent contained in its cavity.
It will be appreciated that, for such constructions, it is possible for corrosion of the magnesium alloy tube to proceed not only from an exposed end but also from its peripheral surface. Such corrosion can lead to a weakening of the overall structure or a possibility of release of biological agents within the cavity of the tube earlier than intended. In order to prevent this the exterior surface of the magnesium tube can be provided with an external protective coating. This coating should resist corrosion by rumen juices and may be, for example of synthetic plastic material. In a known embodiment there is provided a plurality of plastic rings or tyres each extending around the exterior surface of the magnesium tube. In this way as the magnesium tube degrades from an exposed end successive rings or tyres drop away to allow access for the rumen juices to the tube and the tablet enclosed therein.
One known way of controlling corrosion of a magnesium alloy component is described in European Patent Application No. 0 284 258. In this application a magnesium alloy tube is galvanically coupled to an electrically conductive component containing material lower in the electrochemical series than magnesium. For example, an electrically conductive material may be dispersed throughout the body or tablets containing the biologically active material. In this way galvanic corrosion of the magnesium alloy proceeds at a predetermined and measurable rate so that the pellet may be constructed to dispense a biologically active material at predetermined intervals.
However, in order to prevent galvanic corrosion from corroding or otherwise weakening the exterior surface of a tube it is again necessary to provide either an epoxy resin coating or a plurality of non-degradable plastic rings around the exterior of the tube. Such additional components complicate and increase the cost of the manufacturing process of the pellet and limit the configurations which can be used. However, if the contents of the interior of a tubular bolus are to be dispensed at the require predetermined intervals the exterior surface of the tubular component must be protected against corrosion.
It would be an advantage to provide a pellet having a tubular magnesium alloy component in which the corrosion rate of the exterior surface is controllable without resort to an additional protective sleeve or coating.